The Aldens are taking a bicycle trip to visit Aunt Jane and Uncle Andy. Along the way the Boxcar Children find themselves confronted during a storm with a new mystery....

Long Synopsis:

From the back of the book:
Henry, Jessie, Violet and Benny used to live alone in a boxcar. Now they have a home with their grandfather and are going on a bicycle trip.
On the first night of their journey, the Boxcar Children get caught in a rainstorm. They find shelter in an old abandoned house-and a mystery finds them!
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From inside the book:
About This Alden Mystery
Whenever the Aldens-Henry, Jessie, Violet, and Benny-start a trip, they expect some sort of adventure or mystery. It all began when they ran away from an orphanage and made a home for themselves in an old boxcar. Of course the best part of that adventure was finding Grandfather Alden.
When the bicycle trip was first planned, Benny hoped it would mean another adventure. But he didn't dream that on a rainy morning in an empty old farmhouse a mystery would find him. That's the way it happened, and the Aldens found themselves in the middle of a puzzle before they knew it.